In almost all known switch devices having a movable and a stationary contact, the movable contact bounces off the stationary contact upon closure. In piezoelectric bender switches such contact bounce is particularly undesirable for many reasons, including the fact that arcing may occur between the stationary and movable contacts, thereby eroding contact surfaces and effecting the integrity of the switch. In some instances, arcing may cause the stationary and movable contacts to become welded together, thus terminating the useful life of the switching device.
A piezoelectric bender switch generally includes a central metal foil sandwiched between first and second thin layers of piezoelectric ceramic, which in turn are sandwiched between a pair of first and second electrically conductive metal layers. These layers form a beam, which may typically be about one inch in length and one half inch or less in width. One end of the beam is affixed to a support while the free end of the beam carries a contact, referred to herein as a movable contact, wich is mounted on and extends from the first metal layer. The movable contact is insulated from the beam, and a separate stationary contact is disposed on a fixed surface opposite and spaced from the movable contact.
In one mode of operation, a DC voltage, referred to herein as a charging voltage, is applied between the first electrically conductive metal layer and the metal foil. The metal foil and the second metal layer are connected to ground. The charging voltage produces a dimensional change in the first piezoelectric layer, while the second piezoelectric layer and the central foil do not change dimensionally. The dimensional change in the first layer causes a warping of the beam, much like the action of bimetallic strip exposed to a temperature change. The dimensional change in the beam, however, is brought about by the application of a voltage rather than by a change in temperature. The dimensional change brought about by voltage is much more rapid than the dimensional change of a bimetallic strip brought about by temperature.
With relative rapid excitation, the bending or warping motion of the beam is not unidirectional. Rather, the beam oscillates as it carries the movable contact towards the fixed contact. When the charging voltage applied to the first piezoelectrid layer becomes sufficiently large, the resulting displacement of the free end of the beam causes the movable contact to strike the fixed contact. The movable contact, at this time, may be moving at a significant velocity on the order of 10 cm/sec. At contact closure, it is essential that a voltage be applied which is greater than the minimum required voltage to bring the contacts together. A voltage greater than the minimum voltage assures that the contacts will be held together with sufficient force to maintain a good electrical connection.
With known systems, contact bounce occurs when the movable contact, moving at a high velocity, impacts upon the stationary contact. In this condition, the beam oscillations tend to further increase the duration and magnitude of the contact bounce. Specifically, the phase of oscillation of the movable contact may be such that at contact closure, the movable contact may begin moving away from the fixed contact. Also, with systems of this kind, it is common to arbitrarily select the charging voltage to have a fixed magnitude which typically is two or three times the voltage required to bring the contacts together. This arbitrarily chosen charging voltage may cause contact closure at excessive velocity which tends to increase contact bounce still further.
Known systems for suppresing contact bounce include a spring-mounted mass located between the stationary contact and the surface to which the stationary contact is mounted. When the movable contact carried on the free end of the flexible beam impacts upon the stationary contact, the spring absorbs some of the impact. The use of a spring-mounted mass system to reduce contact bounce adds to the cost of manufacturing and constructing switches. Further, such a technique does not necessarily minimize contact bounce. In some cases, the spring-mounted mass, if not properly adjusted, may even exacerbate the contact bounce.